Ultrahigh molecular weight polyethylene (“UHMWPE”) is commonly used in making orthopaedic implants, such as artificial hip joints. In recent years, it has become increasingly apparent that tissue necrosis and osteolysis at the interface of the orthopaedic implant and the host bone are primary contributors to the long-term loosening failure of prosthetic joints. It is generally accepted by orthopaedic surgeons and biomaterials scientists that this tissue necrosis and osteolysis is due, at least in part, to the presence of microscopic particles of UHMWPE produced during the wear of the UHMWPE components. The reaction of the body to these particles includes inflammation and deterioration of the tissues, particularly the bone to which the orthopaedic implant is anchored. Eventually, the orthopaedic implant becomes painfully loose and must be revised.
In order to increase the useful life of orthopaedic implants having UHMWPE parts, several attempts have been made to increase the wear resistance of the UHMWPE, thereby decreasing the number of wear particles that can cause tissue necrosis and/or osteolysis. One method for increasing the wear resistance of UHMWPE utilizes exposure to high-energy radiation, such as gamma radiation, in an inert or reduced-pressure atmosphere to induce cross-linking between the polyethylene molecules. This cross-linking creates a three-dimensional network of polyethylene molecules within the polymer which renders it more resistant to wear, such as adhesive wear. However, the free radicals formed upon irradiation of UHMWPE can also participate in oxidation reactions which reduce the molecular weight of the polymer via chain scission, leading to degradation of mechanical properties, embrittlement, and a significant increase in wear rate. These free radicals are very long-lived (greater than eight years), so that oxidation can continue over a very long period of time resulting in as much as a 5-fold increase in the wear rate as a result of oxidation over a period of about 5 years. Therefore, the long term wear resistance of irradiated UHMWPE, and the useful life of an orthopaedic implant having irradiated UHMWPE parts, substantially depends upon reducing the number of free radicals present in the UHMWPE before it is exposed to an oxidizing environment, such as air or the oxygen-rich in vivo environment.
There are several processes that have been developed to effectively and efficiently reduce the number of free radicals present in irradiated UHMWPE, all of which have met with varying degrees of success. For example, U.S. Pat. No. 5,414,049 discloses a process in which an irradiated formed implant of UHMWPE is heated to a temperature between 37° C. and the melting point of the UHMWPE in an oxygen-reduced, non-reactive atmosphere for a length of time sufficient to reduce the number of free radicals present in the UHMWPE. The disclosed process typically requires at least forty-eight hours (and up to 144 hours) to substantially reduce the number of free radicals. While the process does reduce the number of free radicals contained within the UHMWPE, there can be significant costs associated with heating the irradiated formed implant of UHMWPE for such an extended period of time.
A need therefore exists for a process for effectively and rapidly quenching the free radicals present in irradiated UHMWPE. The invention provides such a process. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.